Tomorrow is my first 747 flight with British Airways, it's BOS-LHR on a 742 . Most 742s and 741s have spiral stair cases, but there are some exceptions. All 744s have the straight staircase. But I'm not sure about 743s.

There is a third option for the -100, -200, and -SP models...an "L" shaped staircase located in the same spot as the circular staircase. Displaced one (or sometimes two) First Class seats, therefore considered un-economical (albeit it was the safest of the "classic" staircases).

We had a similar topic a couple of weeks ago.
There's 2 different staircases on a Classic 100/200/sp :
spiral or straight 'L' shaped, while the last one is quite rare (Braniff had them and seems that Iran Air as well).
I guess that KLM's 200s SUD were retrofitted with a straight staircase when converted to SUD.